Effects of major ions on natural benthic communities: an experimental assessment of the US Environmental Protection Agency aquatic life benchmark for conductivity

Abstract

AbstractElevated concentrations of HCO3−, Cl−, SO42−, Mg2+, Na+, and Ca2+ in freshwater ecosystems are often associated with anthropogenic disturbances. The US Environmental Protection Agency developed a field-based specific conductance (SC) benchmark of 300 µS/cm for streams affected by mountain-top mining operations. The benchmark has been criticized because of the potential influence of confounding variables and difficulty in demonstrating a causal relationship between elevated SC and macroinvertebrate responses. We conducted 4 stream mesocosm experiments to quantify the effects of major ions on aquatic insect assemblages. We exposed insects from streams with low (60–72 µS/cm) and moderate (200–250 µS/cm) SC to major ions at values bracketing 300 µS/cm. We measured community metabolism, macroinvertebrate drift, community composition, and survival. Sixty-six taxa were exposed to NaHCO3, MgSO4, and NaCl in 4 mesocosm experiments, and 8 dominant families/subfamilies occurred in sufficient densities to dev...